1. Field of the Invention
The present invention relates to the heat treatment of materials in an artificial atmosphere. More specifically, the present invention relates to the heat treatment of metals and alloys in an atmosphere substantially purged of oxygen through the use of a bi-phasic cryogen.
2. Description of the Related Technology
The production of finished metal products is carried out through a series of heat treating processes. Extracted raw metal ores are generally heated in furnaces in which ore reduction and smelting take place. Heating the materials into molten form allows the metal to be separated from impurities and allows the molten metal to be uniformly blended with other materials and metal to form alloys and metals of different grades. Once a desired composition is achieved the molten metal is removed from the furnace and allowed to cool in the form of ingots or slabs.
The ingots and slabs are then processed into the desired product form and shape, i.e., bar, sheet, strip, tube, wire. The typical forming and shaping process is generally carried out in a rolling mill furnace. In a rolling mill, ingots and slabs are heated so as to become more malleable and thereby more easily shaped into the desired product form. The heated ingots and slabs are then rolled, i.e., they are passed between opposed rolls in the cavity of the mill whereby they undergo an increase in length and a reduction in height or depth. Generally, it is not possible to reduce large slabs of metal into desired product form by a single pass through a pair of rolls. The forming process usually requires passing the metal several times through the same pair of rolls, wherein the rolls are progressively brought into abutment and the product is brought into its final shape. Alternatively, metals can be passed through a rolling train, wherein a series of rolls with gaps of diminishing width are provided in a successive relationship that conclude with the product being pressed into its final product shape.
Other forming and shaping processes in the art that generally require the heat treating of materials in furnaces include, but are not limited to, sintering powders, brazing metals and sealing glass to metals. As understood by one of ordinary skill in the art, an oxide layer (i.e. mill scale) is formed on the surface of oxidizable materials, particularly metals and alloys, whenever such a material is heat-treated in the presence of oxygen. This oxide layer must be removed, or preferably prevented from forming, before any successive forming or subsequent processing steps can be performed.
Accordingly, there has been a long-felt, yet unresolved, need in the art of metal fabrication to provide a method and apparatus for heat treating metals and alloys that reduces or prevents the formation of an oxide layer on the treated material""s surface. This need is particularly acute in the annealing process, especially in the annealing of exotic metals and alloys. By xe2x80x9cexotic,xe2x80x9d it is meant those comparatively rare specialty metals and alloys that may be particularly susceptible to oxidation, or otherwise have a high affinity for oxygen. Representative exotic metals include, but are by no means limited to, zirconium, titanium, molybdenum, tantalum and columbium.
Annealing is the process through which stresses and distortions in formed metal products are removed. Annealing generally involves the heating of a product to an effective temperature for a period long enough to allow the molecular structure of the material to adjust to a more uniform arrangement, and then controlling the cooling of the material such that the uniform arrangement can be maintained in the final product. Annealing is an important step in the finishing process of metal products. It is through annealing that a uniform and strong product being substantially free of weak spots and distortions is ensured.
Annealing of metal products generally involves several heating and cooling cycles to ensure uniformity of the finished product. As will be appreciated by one of ordinary skill in the art, each such cycle involves passing the metal product through the chamber of a furnace. The presence of oxygen in the furnace results in the formation of an oxide layer on the product""s surface with each pass through the furnace. This layer must be removed from the product before the product can be sent through the furnace for the next heating and cooling cycle.
Removal of the oxide layer generally involves submerging the metal product in an acid bath to remove the oxide layer by corrosion. This xe2x80x9cpicklingxe2x80x9d process necessitates the use of large volumes of acids, such as sulfuric acid, nitric acid and hydrofluoric acid. The presence and use of these acids on-site poses significant health, safety and environmental concerns. The acids must be shipped, delivered, stored and used in large quantities. In addition, pollution control and disposal of these acids is also of great concern and a considerable operating expense. Accordingly, there has been a long-felt need in the art to devise a method and apparatus that allows for the reduction or elimination of the need to pickle products during annealing and finishing processes. A similar need exists in other heat treating processes that ultimately result in the need to pickle products before successive or subsequent processing and finishing operations can be undertaken.
Prior art methods have failed to satisfy these long-felt needs. One such method prescribes the use of a completely fluid tight furnace chamber. The furnace chamber is then vacuum evacuated of substantially all ambient oxygen prior to heating the material to be treated. This process requires a special vacuum furnace and is generally only suitable for small batch processes. Further, the furnace must be capable of preventing the leaching of outside ambient air into the process in order to prevent a corrupting of the entire process. The use of a vacuum furnace also results in the need for a substantially long cooling period which lowers plant productivity. In addition, a vacuum process can be prohibitively expensive for many metals. Estimates on the price of operating a vacuum furnace range from $400-$600 per hour. Thus, there remains a need in the art for a less expensive, non-vacuum process that is suitable for large volume, continuous annealing and heat-treating processes.
Another common prior art method involves the purging of ambient oxygen from the furnace chamber by the introduction of an inert gas blanket. This method requires a continuous flow of gas to provide enough gas pressure in the chamber to prevent the ambient, oxygen rich air from reentering the chamber area. Even with a substantially fluid tight chamber, this process requires an extraordinarily large volume of gas to be used during the process and yet still fails to keep the concentration of residual oxygen low enough to prevent the formation of an oxide layer on most metal products. This is particularly true with respect to the easily oxidizable specialty metals, which still must undergo acid pickling despite the use of inert gases. Thus, there still remains a need in the art to achieve low residual oxygen concentrations through a purging process without having to use substantial volumes of inert gases or reach excessive pressures.
The present invention overcomes the practical problems described above and offers new advantages as well. The present invention is based on the discovery that, quite unexpectedly, the introduction of an inert gas in at least partially liquid form into the heating chamber of a heat treating apparatus produces such an effective blanket purging environment that the residual oxygen concentration, if any, is kept at such a low level that the formation of an oxide layer on a heat treated surface is almost, or completely, non-existent. This is true even when the product being treated is an exotic metal or alloy. Although not wishing to be bound by theory, it is believed that these unexpected results are due to the inherent ability of the transformation of the liquid constituent into gaseous form to achieve high concentrations of the purge gas through volumetric expansion in a desired location; whereas, by contrast, the simple introduction of inert gases, even in large volumes, dissipates before achieving similar concentrations.
Accordingly, one object of the present invention is to provide a heat-treating chamber capable of receiving a gas in at least partially liquified form. It is another object of the invention to provide a heat-treating chamber capable of receiving a gas in at least partially liquified form from a plurality of sources, whereby different gases, or a combination of the same or different gases, can be introduced, simultaneously or at different times, into the same chamber in partially liquified form. It is yet another object of the invention to provide a method of heat-treating a material in a reduced oxygen atmosphere by introducing a purge gas, or purge gases, in at least partially liquefied form into the atmosphere of a heat-treating chamber.
In accordance with an object of the invention, there is provided an apparatus for heat-treating a material comprising a furnace having a sidewall defining a chamber and defining a discharge receiving orifice, and a cryogen source having an outlet in fluid communication with the orifice. In accordance with one aspect of the invention, the furnace may include an untreated product inlet for receiving a product to be heat-treated and a treated product outlet for discharging the product after heat-treating. The product inlet and product outlet may be positioned such that the product enters the furnace through the product inlet, passes through the chamber, and then exits the furnace through product outlet.
In accordance with another aspect of the invention, the chamber may be partially or substantially isolated from the ambient atmosphere outside the furnace. The chamber may also include a hot/work zone wherein a heat source heats a product passing therethrough to a desired, elevated temperature, and a cooling zone wherein a product exiting the hot/work zone is cooled prior to exiting the furnace. The heat source may comprise hot gas jets disposed in the hot/work zone or a heat source which provides heat to the hot/work zone by convection or conduction. The cooling zone may have cooling gas jets disposed therein, provide quenching, or comprise an isolated area for natural cooling from heat transfer with the zone""s atmosphere.
In accordance with another aspect of the invention, the cryogen source may be a low pressure source comprising an inert gas liquified under pressure. The cryogen source may have an outlet and a regulator coupled thereto. The pressure of the cryogen source may be between about 20 to 40 psig. The cryogen may be liquid nitrogen or liquid argon. The cryogen may enter the furnace in bi-phasic form as a spray heavy with liquid. The bi-phasic ratio of liquid to gas may be any effective ratio. Effective ratios may be between about 30/70 liquid to gas to about 90/10 liquid to gas. The ratio may depend on the product being treated and the specific heat-treating process being undertaken.
In accordance with yet another aspect of the invention, there is provided a conduit for providing fluid communication from the cryogen outlet to the discharge receiving orifice. The conduit may be constructed of any material capable of accepting and discharging the cryogen flow. The conduit may comprise 304 grade stainless steel or like materials that can withstand the operating temperatures, pressures and flow rates of the present invention. The conduit may further include a discharge tip. The discharge tip may simply comprise the discharge end of the conduit being tapered or crimped into a slot or other geometric shape which is capable of ensuring a substantially uniform flow of the bi-phasic cryogen into the furnace. Alternatively, the conduit may be fitted with a specialized nozzle which ensures a substantially uniform flow. The conduit and the orifice may be sealed in fluid tight communication or of an integral construction.
According to a further aspect of the invention, there is provided a fluid control means for controlling the flow of cryogen exiting the cryogen source and entering the furnace. The fluid control means may comprise a pump. The pump may be of the venturi-type. The fluid control means may be capable of adjusting the cryogen flow whereby a desired flow rate and/or gas concentration can be regulated.
In accordance with another object of the invention, there is disclosed a method of heat-treating a material in a reduced oxygen atmosphere by the introduction of a bi-phasic cryogen to create a substantially oxygen free atmosphere in a heat-treating chamber.
These and other objects, aspects, features and advantages of the present invention will be apparent from the following description of the invention with reference to the accompanying drawings.